The mobile phone industry has had an enormous development both regarding quality of service and transmission capabilities, as well as the technology for producing advanced communications terminals. In only a couple of decades the communication systems have gone from analogue to digital, and at the same time the dimensions of the communication terminals have gone from briefcase size to the pocket size phones of today. Still today, mobile phones are getting smaller and smaller and the size is generally considered to be an important actor for the end customer. The development in electronics has made it possible to miniaturise the components of the terminals, at the same time making the terminals capable of performing more advanced functions and services. The development of new transmission schemes, the so-called 3rd generation mobile system standing at the door and the 4th generation to be expected maybe ten years later, also provides the possibility to convey more advanced data to the wireless communication terminals, such as real time video.
In order to benefit from the capabilities of the transmission system, the future generation of phones will have a large display in order to perform multimedia input and output, and the picture quality will be of high importance. Still, the keypad or keyboard must be large enough to allow people to press one key at the time. With the development of the communication systems and the capabilities of multimedia functions, the demands on the sound quality from the terminals will also increase. In the next generation of terminals using W-CDMA technique, it will be possible to use a wider audio frequency band. The speaker of the terminal ray also be used as a ringer instead of a traditional buzzer, for a single tone or polyphonic ring signals, and as a hand free speaker built-in into the phone. Normally, these functions are performed by a second speaker mounted in the back of the phone, but combined speakers for two or all three of these functions exist.
In a conventional state of the art communication terminal a terminal audio output is mounted on the front of the terminal above the display with the speaker immediately behind the output. This means that even with a smallest available speaker used today, with a diameter of about 13 mm, the terminal would be approximately this much longer in its longitudinal extension than what would have been the case if only the display and/or keypad had defined the length of the terminal. The antenna of the terminal is generally located at the upper portion of the backside of the terminal, the lower backside being occupied by a battery. Placing the speaker behind the display would therefore increase the thickness of the terminal 1 since it would have to compete with the antenna for the relevant space.
The performance of the speaker is to a high extent dependent on the back volume, i.e. the acoustic resonance cavity behind the speaker. For best performance, the back volume should be sealed. In a normal case the whole phone is used as back volume, but it is almost possible to seal a phone. As mentioned above, almost every phone today has the speaker position in the upper parts of the front of the terminal above the display and keyboard. This means that the height of the speaker, display and the keyboard determines the length of the phone. Since the speaker performances is highly dependent on the dimension, reduction of the speaker size results in less good acoustics. There is also a general desire to have larger displays in the communication terminal, not just wider but also longer, and with the common layout of the keyboard which people are used to, the height of the terminal can basically not be reduced more than today without resulting drawbacks to the display or keypad.
In US 2002/0137478, assigned to NEC Viewtechnology, a design is proposed in which the speaker and also the microphone are moved to the back side of a terminal. The publication mainly relates to a so called clamshell design, but is equally applicable to a standard stick-type terminal. The speaker and the microphone are devised as identical flat panel type speakers. When using the terminal for voice communication the upper speaker acts as a normal terminal speaker, and the lower speaker is used in a reversed function as a microphone. Furthermore, the two speakers can be used for reproducing stereophonic sound from e.g. FM radio stations. By placing the upper speaker on the back side of the terminal, the length of the terminal may be reduced, or alternatively the display may be increased. However, the thickness of the terminal is instead increased, since the speaker is placed behind the display instead of above it.